Why aren't photos being deleted from my iPad?

Hi,

so my iPad is full and I am trying to remove some files. Ideally, I would like to remove the offline copies of some of my photos, as these are in my iCloud (with plenty of storage) and the downloaded ones use up about 5 GB of space. I have enabled the "Optimise my iPad storage" option, but it hasen't actually deleted any of the offline pictures, even though the iPad is full and I have been promted to deleted some stuff. What to do?